The research aims to determine how intestinal microvillus membranes function at a molecular level to control the digestion and absorption of essential nutrients. The "microviscosity" of isolated microvillus membranes is relatively high and they show a thermotropic phase transition. Fluorescence polarization, using the hydrocarbon probe diphenylhexatriene and other fluorophors, will be applied to study the biochemical basis and functional significance of these observations. Isolated lipid and protein fractions will be studied separately and in combination to examine lipid-lipid and lipid-protein interrelationships. Effects of varying the lipid composition by in vivo or in vitro treatments will be explored. The relationship of the thermotropic transition temperature to pH and divalent cation concentration will be determined systematically. Effects of the thermotropic transition on enzymes, transport and surface receptor functions of the membrane will be examined. The thermotropic transition will also be investigated by differential scanning calorimetry, electron spin resonance, freeze-fracture electron microscopy and X-ray diffraction. The effects of pH on microvillus membrane microviscosity will be explored in relation to membrane stability and the pathogenesis of duodenal peptic ulcer. The identification and characterization of microvillus membrane proteins which bind vitamin A will be studied by fluorescence methods. The role of such receptors in trans-membrane transport of the nutrient will be determined.